Direct acting overcurrent system for high voltage circuit breakers

ABSTRACT

A solid state system for high voltage circuit breaker applications utilizing a solid state sensor and time delay circuit adapted to simulate din induction disk type time-current characteristics utilizing standardized components otherwise employed in low voltage breaker applications with adjustable features for tailoring the trip current generated by the solid state circuitry to be compatible with the particular magnetic latch being employed in the high voltage circuit breaker. The magnetic latch is provided with an independent isolated winding controlled by an isolated power source for performing a tripping operation under the control of external and remote control means. Isolation of the sensor and timing circuitry from the control transformers prevents disruptive or damaging transients from being coupled to the solid state sensing circuitry.

United States Patent 91 'Zocholl 1 Mar. 27, 1973 [54] DIRECT ACTINGOVERCURRENT Primary Examiner-James D.v Trammell SYSTEM FOR HIGH VOLTAGEAttrney-Ostrolenk, Faber, Gerb & Soffen CIRCUIT BREAKERS [75] Inventor:Stan!ey E. Zocholl, Holland, Pa. [57] ABSTRACT [73] Assignee: I-T-EImperical Corporation, A fi .state for ig voltage clrcult bdregkerPhiladelphia Pa. app lcations uti lZll'lg a sol state sensor an timedelay circuit adapted to simulate dlIl induction disk [22] Filed: Oct.5, 1971 type time-current characteristics utilizing standardizedcomponents otherwise employed in low voltage [21] Appl' 186672 breakerapplications with adjustable features for tailoring the trip currentgenerated by the solid state [52] US. Cl ..3l7/36 TD,3l7/46 circuitry tobe compatible with the particular mag- [51] Int. Cl. ..H0lh 47/18 neticlatch being employed in the high voltage circuit [58] Field of Search..3l7/36 TD, 46 breaker. The magnetic latch is provided with anindependent isolated winding controlled by an isolated References Cltedpower source for performing a tripping operation UNITED STATES PATENTSunder the control of external and remote control means. isolation of thesensor and timing circuitry 3,434,011 3/1969 Zocholl ..3l7/36 TD fromthe control transformers prevents disruptive or 3,544,846 l2/l970Thompson ..3l7/36 TD damaging transients from being coupled to the solidMathews at a]. state ensing circuitry 3,211,958 /1965 Miller et al..3l7/36 TD 3 Claims, 1 Drawing Figure /i'/ 64 5 A /,5 jg

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Patented March 27, 1973 INVENTOR.

DIRECT ACTING OVERCURRENT SYSTEM FOR HIGH VOLTAGE CIRCUIT BREAKERS Thepresent invention relates to protective circuits for power transmissionand distribution and more particularly to a novel solid state circuitfor use with circuit breakers of high voltage rating with standardizedand discretely adjustable components employed to enable the use of thesolid state timing and pick-up circuits with circuit breaker magneticlatches having different operating requirements.

BACKGROUND OF THE INVENTION Solid state direct acting trip devicesystems have found widespread use in the field of power transmission anddistribution networks whose ratings are such as to require low voltagecircuit breakers. Such systems normally comprise special ratio currenttransformers commonly referred to as current sensors; a solid statetiming and trip circuit; and a magnetic latch, which is preferablyprovided with a spring loaded armature which is released when thesensors generate the requisite trip current. The stored energy of thespring provides the tripping force for operating the circuit breaker.

The solid state timing and pick-up circuit is usually provided with along time delay element and/or a short time delay element as well as aninstantaneous element which are each specifically designed for lowvoltage power system applications. However, direct acting overcurrentsensing systems of the solid state type are not suitable for use in highvoltage breaker applications in their present form.

BRIEF DESCRIPTION OF THE INVENTION Direct acting solid state overcurrenttripping systems constructed for high voltage circuit breaker use mustbe provided with the following elements and characteristics which arerequired for high voltage circuit breaker applications;

a. Asolid state circuit having induction disk type time-currentcharacteristics provided with an adjustable time dial to change thecharacteristic time scale over a ratio of one to 15. Devices of thistype are disclosed in US. Pat. Nos. 3,444,434 issued May 13, 1969;3,434,011 issued Mar. 18, 1969; and 3,319,127 issued May 9, 1967, all ofwhich patents have been assigned to the assignee of the presentinvention. Techniques and circuitry are described in the aforementionedU.S. patents in which solid state circuitry is employed to simulate thedesired time current characteristics otherwise obtainable only throughthe use of electromechanical relay devices.

b. An overcurrent trip system input comprising standard five amperecurrent transformers.

c. A standard tap arrangement including adjustable sociated standardcurrent transformer and comprises tap means for converting the standardfive ampere input to the voltage required by the solid state circuitryand magnetic latch. The intermediate sensors develop the necessaryoutput voltage across burden resistors provided for each phase, whichare, in turn, coupled to full wave bridges adapted to convert the outputsignal to DC. A circuit is provided for establishing a reference levelutilized by a timing and pick-up circuit to control tripping inaccordance with the overcurrent level in a given instant, which iscompared by the timing circuit against the reference level. The delayedoutput signal developed by the timing and pick-up circuits is utilizedto control a silicon controlled rectifier (SCR), which, when triggered,energizes the trip winding of a magnetic latch, which is powered by thesensing circuit. Adjustment to any one of the discrete tap positions ofthe intermediate sensor facilitates dovetailing the system to therequirements of the magnetic latch.

The magnetic latch is provided with an isolated winding coupled in aseparate circuit powered by an independent power source for tripping thebreaker only when closed by a remote relay or other control means. Theuse of intermediate adjustable sensors in conjunction with the standardtransformers provides a wide variety of adjustments to suit theequipment to the particular magnetic latch while at the same timepreventing remote control operation of a breaker at reduced current makeand break levels.

It is, therefore, one object of the present invention to provide a noveldirect acting overcurrent system for high voltage circuit breakers andthe like, which incorporates standardized components while providing forrapid adjustment of the system with magnetic latches of varyingcharacteristics.

BRIEF DESCRIPTION OF THE FIGURES The above, as well as other objects ofthe present invention will become apparent when reading the accompanyingdescription and drawing, in which:

The sole FIGURE shows a schematic diagram of an overcurrent sensingsystem designed in accordance with the principals of the presentinvention.

DETAILED DESCRIPTION OF THE DRAWING The system of the FIGURE shows adirect acting overcurrent relay arrangement utilizing standard fiveampere current transformers 10, 11 and 12 for each of the phases A, Band C of a three-phase power distribution network. Whereas the system tobe described herein is equally advantageous for use in single-phase andmulti-phase systems, the description given hereinbelow will be directedto a three-phase system.

Each of the current transformers l0, l1 and 12 monitors the currents intheir respective phases due to the inductive coupling as shown. Thecurrent transformers are each grounded at one terminal as shown andsupply current to their intermediate current sensors 13, 14 and 15respectively, each of which are provided with a tapped primary winding13a, having adjustably selectable taps l3a-l, 13a-2 and l3a-n through1511-], ISa-n. Manually adjustable switch means may be provided forselection of the taps, or alternatively,

plug-in inserts may be provided to establish an electrical circuitbetween each of the nine grounded terminals of current transformer -12and the primary winding circuits 130-1 through a-n, respectively.

' The intermediate sensors are respectively provided with secondarywindings 13b-15b for developing a voltage across their respective burdenresistors 16 18. The voltage developed across each of the burdenresistors is applied across a first pair of terminals of an associatedfull-wave diode bridge circuit 19 21, respectively. The remaining pairsof terminals of the bridge rectifiers are coupled in parallel to buses33 and 34, respectively. The full-wave bridge rectifiers operate toprovide a D.C. voltage across buses 33 and 34, whose value at any giveninstant is the largest D.C. level supplied by one of the three bridges19-21. The D.C. voltage developed across buses 33 and 34 are utilized toenergize a reference circuit consisting of zener diode 24, which isconnected in series with a diode 22 and current limiting resistor 23. Afilter capacitor 25 is coupled in parallel across diode 24 to filter andstabilize the D.C. level. The resulting fixed voltage V,, serves as areference voltage for pick-up and timing circuit 26, whose necessarybias levels are supplied by the rectified sensor output voltage asrepresented by leads 35- 36.

The timing and pick-up circuit may, for example, be of the typedescribed in the above mentioned U.S. patents. Considering U.S. Pat. No.3,319,127, for example, and specifically FIG. 2 thereof, a pick-upcircuit 12 is provided for preventing the operation of the timing (i.e.,delay) circuit until a minimum pick-up level is received, atwhich timethe time delay circuit is energized.'The timing circuit is designated bynumeral 13 in FIG. 2 of U.S. Pat. No. 3,319,127 and is comprised ofaplurality of resistors and capacitors connected in a predeterminedmanner so as to provide a time-current characteristic which, forexample, simulates the timecurrent characteristics of an induction disktype (electromagnetic) device. After a time delay elapses betweenpick-up and timing out, depending upon the magnitude of the overcurrentcondition being sensed, the output signal appearing in output terminal26A is applied to the trigger electrode of SCR 27, turning the SCR ON.Conduction of SCR 27 establishes a current path through the branchcircuit comprised of SCR 27 and coil 28 of a circuit breaker magneticlatch. Energization of the magnetic latch coil 28 is derived from thesame power utilized to establish the reference voltage and to operatethe timing and pick-up circuits 26. Energization of the magnetic latchcoil is mechanically coupled to circuit breaker 52 as shown by dottedline 53 to activate the circuit breaker trip circuit and trip open allthree phases simultaneously.

The circuit breaker magnetic latch is provided with a second isolatedcoil 29, which is connected in a single loop circuit comprising abattery source 32, an external trip contact set 30, and a breakerauxiliary contact set 31. Contact set 30, which is normally open,represents any desired external trip contact such as a control switch oran external contact. Contact set 31 forms part of the circuit breaker 52and is open when the circuit breaker is opened and closed when thecircuit breaker is closed (i.e., engaged). Remote control operationwhich may be provided for any desired purpose is activated by closure ofcontact set 30. Thus, when the circuit breaker is in the closedcondition, contact set 31 is closed establishing a current path throughthe single loop circuit. The current flows through isolated coil 29,which magnetically engages the spring loaded armature (not shown forpurposes of simplicity) which, in turn, is

mechanically coupled (see dashed line 53) to the circuit breaker tripmechanism to cause simultaneous opening of the three phases.

The time-current characteristic of the timing and pickup circuitry 26 ispreferably provided with an adjustable time dial for changing thecharacteristic time scale over a ratio of 1:15. A suitable arrangementwhich may be employed in the timing circuit of the present applicationis described in U.S. Pat. No. 3,444,434, where a variable resistor isused in series with a capacitor forming a simple series RC timingcircuit. An alternate time dial arrangement is shown in U.S. Pat. No.3,573,555 where a variable duty cycle clamp circuit is used when thetiming circuit is more complex.

A salient feature and advantage of the present system resides in thefact that no separate control power is required for overcurrent trippingsince the same D.C. level utilized as the sensing voltage is employed topower the timing and pick-up circuit 26 to provide the voltage referencelevel V and to energize the coil 28 of the magnetic latch. In addition,the solid state circuitry is isolated from the control power sourcethrough the first and intermediate sensors 10-12 and 13-15 respectively,and bridges 16-18 so as to isolate disruptive and/or damaging transientsfrom the solid state circuitry. The solid state trip circuitry isautomatically adjustable to fit the trip current requirements of themagnetic latch and, therefore, need not be individually designed tohandle a range of standard high current shunt trip coils, dueto theadjustable nature of the intermediate sensors. In addition, the use ofan isolated winding 29 for the circuit breaker magnetic latch requiresonly a small operating current so as to reduce the current make andbreak requirements for external relaying.

It can be seen from the foregoing description that the direct actingovercurrent trip system described hereinabove provides the use ofstandard current transformers, standard relay time-over currentcharacteristics with standard four to one pick-up tapsand time dialadjustments presently required in high voltage power systemapplications, while at the same time providing system components whichare isolated from the control power transients and surges. The use ofsaturating intermediate sensors ofthe type described in U.S. Pat. No.3,434,011 allows secure operation up to one hundred multiples of pick-upcurrent, thus providing stable and reliable system operation.

Although there has been described a preferred embodiment of this novelinvention, many variations and modifications will now be apparent tothose skilled in the art. Therefore, this invention is to be limited,not by the specific disclosure herein, but only by the appending claims.

What is claimed is:

1..A direct acting overcurrent sensing system for operating high voltagecircuit breakers and the like comprising:

a first current sensor inductively coupled to a power line beingprotected;

said first sensor comprising a saturable core adapted to saturate at apoint below the upper end of the operating current range of said system;

a second current sensor coupled to the output of said first currentsensor, said second sensor having inductively coupled primary andsecondary windings;

at least one of said windings having a plurality of taps;

means adjustably coupled to said taps for controlling the output levelof the signal developed by said second sensor to be compatible with theoperating characteristics of the overcurrent sensing system;

bridge means for converting the output of said second sensor into a DCsignal;

means coupled to said bridge means for developing a predeterminedreference level;

a static time delay circuit;

circuit means coupled to said bridge means and said reference levelgenerating means for enabling said time delay circuit when the output ofsaid bridge means reaches said reference level;

said time delay circuit including means for generating a delayedtriggering signal whereby said delay is inversely proportional to themagnitude of the output of said bridge means; I

a branch circuit coupled to said bridge means comprising seriesconnected switch means and a trip coil, said switch means having acontrol input coupled to the output of said time delay circuit forenergizing said coil by said bridge means upon the occurrence of saidtriggering signal;

circuit breaker coupled to said line being protected, said circuitbreaker comprising magnetic latch means for including said trip coil fortripping said circuit breaker when said tripping coil is energized, saidsecond sensor adjustable means being settable to provide the requisiteoperating power to said magnetic latch.

2. The system of claim 1 wherein said second sensor comprises asaturable core adapted to saturate at a point below the upper end of theoperating current range of said system.

3. The system of claim 1 wherein said magnetic latch means is furthercomprised ofa second trip coil;

an independent power source and a first normallyopen contact set;

said circuit breaker including a second contact set which closes oropens when the breaker is respectively closed or opened;

said second coil being connected in a series circuit with said secondcoil, said independent power source and said second contact set;

control means for closing said first contact set when a circuit breakertripping operation is desired independently of an overcurrent conditionby said sensing system, for energizing said second trip coil to tripsaid breaker when said second contact set is closed.

1. A direct acting overcurrent sensing system for operating high voltagecircuit breakers and the like comprising: a first current sensorinductively coupled to a power line being protected; said first sensorcomprising a saturable core adapted to saturate at a point below theupper end of the operating current range of said system; a secondcurrent sensor coupled to the output of said first current sensor, saidsecond sensor having inductively coupled primary and secondary windings;at least one of said windings having a plurality of taps; meansadjustably coupled to said taps for controlling the output level of thesignal developed by said second sensor to be compatible with theoperating characteristics of the overcurrent sensing system; bridgemeans for converting the output of said second sensor into a D.C.signal; means coupled to said bridge means for developing apredetermined reference level; a static time delay circuit; circuitmeans coupled to said bridge means and said reference level generatingmeans for enabling said time delay circuit when the output of saidbridge means reaches said reference level; said time delay circuitincluding means for generating a delayed triggering signal whereby saiddelay is inversely proportional to the magnitude of the output of saidbridge means; a branch circuit coupled to said bridge means comprisingseries connected switch means and a trip coil, said switch meAns havinga control input coupled to the output of said time delay circuit forenergizing said coil by said bridge means upon the occurrence of saidtriggering signal; a circuit breaker coupled to said line beingprotected, said circuit breaker comprising magnetic latch means forincluding said trip coil for tripping said circuit breaker when saidtripping coil is energized, said second sensor adjustable means beingsettable to provide the requisite operating power to said magneticlatch.
 2. The system of claim 1 wherein said second sensor comprises asaturable core adapted to saturate at a point below the upper end of theoperating current range of said system.
 3. The system of claim 1 whereinsaid magnetic latch means is further comprised of a second trip coil; anindependent power source and a first normally-open contact set; saidcircuit breaker including a second contact set which closes or openswhen the breaker is respectively closed or opened; said second coilbeing connected in a series circuit with said second coil, saidindependent power source and said second contact set; control means forclosing said first contact set when a circuit breaker tripping operationis desired independently of an overcurrent condition by said sensingsystem, for energizing said second trip coil to trip said breaker whensaid second contact set is closed.